This invention relates generally to optical films and, more particularly, to polyvinyl alcohol-based films exhibiting improved adhesion to an underlying substrate.
Polarized sunwear, visors, goggles, and window panes are designed to selectively absorb or reflect away horizontally polarized light. They are effective in reducing the polarized glare that can interfere with vision. Such glare is particularly noticeable when reflections occur from expansive flat surfaces such as water or roadways, but also can be a significant problem under hazy conditions such as smoggy or foggy skies. Polarizers offer a unique advantage for improved vision.
Many of the polarizers used in optical devices include polyvinyl alcohol-based films, which have been known and developed over the past 60 years, as evidenced by the considerable patent history including among others, U.S. Pat. Nos. 2,237,567, 4,992,218, 5,071,906, 5,670,092, and 6,583,929 B2. Polyvinyl alcohol-based films can be formulated to have excellent polarizing properties. However, the chemical and physical properties of polyvinyl alcohol-based films remain a challenge for research and development. These films typically are quite thin (less than 100 micrometers thick), and they are easily deformed or ripped in handling. In addition, the base film material and/or the polarizing agents added to it might be sensitive to heat, humidity, photobleaching, or other environmental exposures.
To avoid the risk of degradation from such environmental influences, polyvinyl alcohol polarizing films typically are encapsulated in lenses or window panes. Even then, such films might be susceptible to damage during the process of forming the optical device. For example, the heat of a thermoplastic molding process might degrade the film's polarizing efficiency. Additional challenges are encountered in thermoset lens casting: the fragile and easily displaced film must be protected while evenly delivering and reactively solidifying liquid-phase thermoset material around the polarizer, and care must be taken to ensure that there is adequate adhesion between the polarizer and the thermoset polymer to withstand any subsequent lens processing.
The issue of film adhesion is important, because polyvinyl alcohol polarizing films typically are not intrinsically reactive to further bonding with most desired optical polymer systems. Therefore, the film needs to be changed, treated, or otherwise processed to facilitate reliable adhesion. Various methods have address this need. U.S. Pat. Nos. 4,818,624 and 6,235,396 B1, for example, discuss coatings that can be applied to the polarizing film, for either improved stability to heat and moisture or improved adhesion within standard hard resin thermoset polymers. Such approaches can be successful, but additional coatings entail additional processing steps, with increased cycle time, costs, and possible losses from contamination or process variables. Consequently, alternative approaches are needed.
Another approach is to sandwich the polarizing film between additional, protective polymer layers adhesively bonded to the polarizer. These protective layers can be treated for improved adhesion to the adhesive layers or to subsequent lens materials. This approach is described, for example, in U.S. Pat. Nos. 5,051,309, 6,512,562 B1, and 4,292,370. Again, additional process steps are required to sandwich the film, and final optical properties might be downgraded due to different refractive indices, coefficients of thermal expansion, or other chemical or physical properties of the sandwiching layers and the applied adhesives. In addition, care still must be taken to ensure good adhesion during subsequent processing, such as grinding, edging, or coating a lens. This will be a particular concern with the multi-layer sandwich approach, because there are more discrete layers (typically at least five layers: protective layer/adhesive/polyvinyl alcohol-based polarizer/adhesive/protective layer) that may offer weak interfaces susceptible to delamination. Moreover, because of the added thickness of the sandwich construct, the overall thickness of the final lens might be larger than is cosmetically desirable.
Surface modification via chemical or physical changes of the polyvinyl alcohol-based film can afford an alternative and beneficial approach to enhanced subsequent adhesion. Such surface modifications often are encountered in other disciplines, for other types of materials, such as silicon wafer pre-treatments for semiconductor processing, or treatments with silicon- or fluorine-based organochemistry to create hydrophobic or anti-stick surfaces. U.S. Pat. Nos. 6,220,703 B1 and 6,413,641 B1 describe surface treatments and specific surface modifications of polarizing films comprising polyethylene terephthalate that lead to improved adhesion. However, given the distinct differences in chemical and physical properties between polyvinyl alcohol and these other materials, it is likely that different treatments will be required to achieve improved adhesion to polyvinyl alcohol-based films.
It should, therefore, be appreciated that there remains a need for, improved methods to enhance adhesion of polyvinyl alcohol-based polarizers in optical constructs. Particularly useful would be methods that reduce the number of process steps, or cycle time of production. The present invention fulfills these needs and provides further advantages.